1. Field of the Invention
The invention is related to the field of magnetic disk drive systems and, in particular, to a perpendicular magnetic recording medium having an interlayer of a non-magnetic Nickel (Ni) alloy.
2. Statement of the Problem
One type of recording medium currently used in magnetic recording/reproducing apparatuses is a longitudinal magnetic recording medium. A longitudinal magnetic recording medium includes a magnetic recording layer having an easy axis of magnetization parallel to the substrate. The easy axis of magnetization is the crystal axis that is aligned along the lowest energy direction for the magnetic moment. Another type of recording medium is a perpendicular magnetic recording medium. A perpendicular magnetic recording medium includes a magnetic recording layer having an easy axis of magnetization oriented substantially perpendicular to the substrate. Hexagonal Close Packed (HCP) Co-alloys are typically used as the magnetic recording layer for both longitudinal and perpendicular recording. The easy axis of magnetization for these materials lies along the c-axis or <0001> direction.
The perpendicular magnetic recording medium is generally formed with a substrate, a soft magnetic underlayer (SUL), an interlayer, an underlayer, a perpendicular magnetic recording layer made of a hard magnetic material, and a protective layer for protecting the surface of the perpendicular magnetic recording layer. The SUL serves to concentrate a magnetic flux generated from a magnetic head and to serve as a flux return path back to the return pole of the head during recording on the magnetic recording layer. The underlayer and the interlayer serve to control the size of magnetic crystal grains and the orientation of the magnetic crystal grains in the magnetic recording layer. The underlayer and the interlayer also serve to magnetically de-couple the SUL and the perpendicular magnetic recording layer.
The interlayer above the SUL may be an amorphous material or a crystalline material with a Face Centered Cubic (FCC) or HCP structure. For example, Ni or Cu alloys such as NiFe or CuNb can be used for the interlayer. The underlayer above the interlayer is typically made from Ruthenium (Ru) or a Ru-alloy. One problem with many current layer structures for perpendicular magnetic recording media is that if the interlayer is made of a material that is ferromagnetic, the material can couple with the SUL and increase the media noise. It is also desirable for the interlayer to give rise to small, uniform grains and strong perpendicular c-axis orientation in the underlayer and magnetic layer. This creates maximum signal-to-noise ratio (SNR) and high coercivity (Hc) for the medium.